The present invention relates to a washing machine, and more particularly, to a ball balancer for a washing machine for preventing an unbalanced rotation of a rotary tub generated due to unbalanced laundry in the rotary tub to reduce vibrations and noises.
A conventional washing machine is provided with an outer tub supported by a suspension inside an external cabinet and a rotary tub or spin basket rotatably installed in the outer tub for containing the laundry. The washing machine generally performs washing, rinsing and dehydrating operations according to a controlling program stored in a controller. During the dehydrating operation of laundry contained within the rotary tub, the rotary tub rotates at a high speed so that washing water is dehydrated from the laundry by a rotational force of the rotary tub.
However, an unbalanced load of the laundry within the rotary tub makes the rotary tub rotate in an unbalanced state. Such an unbalanced rotation of the rotary tub is severe, especially at an initial stage of the dehydrating operation and, at this time, vibrations and noises are generated in the outer tub. Thus, a balancer is installed on the rotary tub to prevent the above vibrations and noises. Various balancers are proposed and selected according to the characteristics of the washing machine. U.S. Pat. No. 4,433,592 classifies the types of balancers into a liquid balancer, a solid balancer and a ball balancer.
The ball balancer has a multiplicity of balancing balls serving as a balancing weight and is widely used for a washing machine because of a superior balancing effect compared to the others. FIG. 8 shows a schematic section of a rotary tub 111 provided with a conventional ball balancer 101. As shown in FIG. 8, the conventional ball balancer 101 includes an annular casing 103 having an annular receiving chamber 105 therein, and fixedly installed at the upper portion of the rotary tub 111. The receiving chamber 105 of annular casing 103 is provided with a multiplicity of balancing balls 107 and a viscous fluid 109. The balancing balls 107 made of steel are immersed in the viscous fluid 109. The balancing balls 107 and viscous fluid 109 in the receiving chamber are circumferentially movable within the receiving chamber 105.
The rotation of the rotary tub 111 generates centrifugal force proportional to a square of the rotation speed of the rotary tub. Since the center of the gravity of an unbalanced rotary tub 111 is eccentric relative to the rotation axis thereof, the balancing balls 107 in the receiving chamber 105 move in the receiving chamber 105 opposite to the unbalanced load of the laundry by the action of the centrifugal force while the rotary tub 111 rotates. Accordingly, the balancing balls 107 compensate for the unbalanced load of the laundry to balance the rotary tub 111, so that the vibrations of the rotary tub 111 can be suppressed.
However, in the conventional ball balancer 101 for the washing machine, when the balancing balls 107 within receiving chamber 105 are moved by the centrifugal force toward the opposite side of the unbalanced laundry, a concentrated pressure due to the centrifugal force is applied by the balls on the inner surface of the radial outer wall of the casing 103. Thus, a stress concentration due to the pressure of the balancing balls 107 is generated on the outer wall of the casing 103 which, over a period of time, will cause deformation of the annular casing 103.
Moreover, since the conventional ball balancer 101 is combined with a vertically short upper portion of the upper circumference of the rotary tub 111, the radial force of the balancing balls 107, i.e., the strong pressure applied to the radial outer wall of the casing 103, is transmitted to that short upper portion of the rotary tub 111, to thereby cause deformation to the upper portion of the rotary tub 111 where the ball balancer 101 is installed. Particularly, in case that the rotation speed of the rotary tub 111 is increased to enhance the dehydrating efficiency, the upper portion of the rotary tub 111 may be damaged by the centrifugal force which increases in proportion to a square of the rotation speed. In this case, the balancing balls 107 may break away causing the viscous fluid 109 to flow out from the ball balancer 101, so that the ball balancer 101 does not perform its balancing function stably, thereby generating vibrations and noises to the rotary tub 111.